1. INTRODUCTION Internal combustion engines are devices that generate work using the products of combustion as the working fluid rather than as a heat transfer medium. To produce work, the combustion is carried out in a manner that produces high-pressure combustion products that can be expanded through a turbine or piston. The engineering of these high pressure systems introduces a number of features that profoundly influence the formation of pollutants. There are three major types of internal combustion engines in use today: (1) the spark ignition engine, which is used primarily in automobiles; (2) the diesel engine, which is used in large vehicles and industrial systems where the improvements in cycle efficiency make it advantageous over the more compact and lighter-weight spark ignition engine; and (3) the gas turbine, which is used in aircraft due to its high power/weight ratio and also is used for stationary power generation. Internal combustion engines are most commonly used for mobile propulsion in vehicles and portable machinery. In mobile equipment, internal combustion is advantageous since it can provide high power-to-weight ratios together with excellent fuel energy density. Generally using fossil fuel (mainly petroleum), these engines have appeared in transport in almost all vehicles (automobiles, trucks, motorcycles, boats, and in a wide variety of aircraft and locomotives). Aim of this experiment is to determine performance characteristics of a four stroke diesel engine. 1.1. The Four Stroke Diesel Engine The four-stroke diesel engine is similar to the four stroke gasoline engine. They both follow an operating cycle that consist of intake, compression, power, and exhaust strokes. They also share similar systems for intake and exhaust valves. A diesel engine is much more efficient than a gasoline engine, such as the diesel engine does not require an ignition system due to the heat generated by the higher compression, the diesel engine has a better fuel economy due to the complete burning of the fuel, and the diesel engine develops greater torque due to the power developed from the high-compression ratio.
Intake Stroke: The piston is at top dead center at the beginning of the intake stroke, and, as the piston moves downward, the intake valve opens. The downward movement of the piston draws air into the cylinder, and, as the piston reaches bottom dead center, the intake valve closes. Figure 1. Induction stroke Compression Stroke: The piston is at bottom dead center at the beginning of the compression stroke, and, as the piston moves upward, the air compresses. As the piston reaches top dead center, the compression stroke ends. Figure 2. Compression stroke Power Stroke: The piston begins the power stroke at top dead center. At this point, fuel is injected into the combustion chamber and is ignited by the heat of the compression. This begins the power stroke. The expanding force of the burning gases pushes the piston downward, providing power to the crankshaft. The diesel fuel will continue to bum through the entire power stroke (a more complete burning of the fuel). The gasoline engine has a power stroke with rapid combustion in the beginning, but little to no combustion at the end.
Figure 3. Power stroke Exhaust Stroke: As the piston reaches bottom dead center on the power stroke, the power stroke ends and the exhaust stroke begins. The exhaust valve opens, and, as the piston rises towards top dead center, the burnt gases are pushed out through the exhaust port. As the piston reaches top dead center, the exhaust valve closes and the intake valve opens. The engine is now ready to begin another operating cycle. Figure 4. Exhaust stroke The diesel internal combustion engine differs from the gasoline powered Otto cycle by using a higher compression of the fuel to ignite the fuel rather than using a spark plug ("compression ignition" rather than "spark ignition"). In the diesel engine, air is compressed adiabatically with a compression ratio typically between 15 and 20. This compression raises the temperature to the ignition temperature of the fuel mixture which is formed by injecting fuel once the air is compressed. The ideal air-standard cycle is modeled as a reversible adiabatic compression followed by a constant pressure combustion process, then an adiabatic
expansion as a power stroke and an isovolumetric exhaust. A new air charge is taken in at the end of the exhaust, as indicated by the processes a-e-a on the diagram. Figure 5. Air standard diesel engine cycle The main difference between the Diesel and Otto engine is burning of the fuel. In a Gasoline engine, the air/fuel mixture enters the cylinder and creates a stoichiometric homogeneous mixture, which is ignited and the flame travels from the spark and outwards to the liner. In the Diesel engine, air enters the cylinder, fuel is injected, self-ignites and burns with a diffusion type of combustion. The Diesel has: Higher compression ratio (higher thermodynamic efficiency, no knock) No throttle (no pumping losses, power fuel) Combustion is always lean (lower heat losses, higher efficiency) More flexible about choice of fuel
2. EXPERIMENTAL CALCULATIONS 2.1 Torque and Power Output The engine torque was measured by the help of a hydraulic dynamometer. The power output is calculated from the torque by multiplying by the angular velocity in radians per second. Because the dynamometer acts as a brake on the engine, the power at the output shaft is referred as the Brake Power (P B ). where: (1) is brake power (W), N is speed (rpm), is torque (Nm). 2.2. The Performance of an Ideal Engine One aspect of engine testing is to determine how the torque and brake power vary with engine speed. To interrupt the results from a real engine, it is necessary to establish the maximum performance that can be expected from an ideal engine, which converts the energy contained in the fuel into mechanical work without loss. The power output depends on the rate where the fuel can be burned. For complete combustion, the fuel must be mixed with air in the correct chemical proportions. If sufficient oxygen is available, a hydrocarbon fuel can be completely oxidized. The carbon in the fuel is then converted to carbon dioxide CO 2, and the hydrogen to water H 2 0. The amount of air drawn into the cylinder that determines how much fuel can be burned during each cycle. Ignoring the volume occupied by the fuel, the volume of air induced into the cylinder during each cycle is ideally equal to swept volume. If the air drawn in from atmosphere at a density ( ), then: (2) where: is density of air (kg/m 3 ) is swept volume (m 3 ) The air consumption rate is given by formula: (3) where: is air consumption rate (kg/s), N is speed (rpm),
n=1 for two stroke engine n=2 for four stroke engine Assuming complete combustion, heat generated per unit mass of fuel is equal to the calorific values, H. This is typically 42000 kj/kg for petrol, 39000 kj/kg for diesel fuel. The rate, Q at which heat is supplied to the engine, is given by: (4) All the energy could be converted into mechanical power, the power output would be: (5) Expressing the fuel consumption rate in term of the other variables given by formula 3,4,5 Ideal Brake Power Output: (6) 2.3. Brake Mean Effective Pressure and Specific Fuel Consumption Another measure of engine efficiency are brake mean effective pressure (bmep) and specific fuel consumption (sfc). (kpa) (7) (g/kwh) (8) Specific fuel consumption is a useful measure of engine performance because it relates directly to the economy of an engine. It enables the operator to calculate how much fuel is required to produce a certain power output for a certain power output for a certain length of time. So the specific fuel consumption can be used to estimate the economic performance of the different engine type. 2.4. Thermal Efficiency In thermodynamics, the thermal efficiency ( ) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a steam turbine or a steam engine, a boiler, a furnace, or a refrigerator for example. The thermal efficiency is defined as: (7) When we expand the terms in, we can obtain following equation: (8) where;
α is the cut-off ratio (ratio between the end and start volume for the combustion phase) r is the compression ratio ( ) is ratio of specific heats ( ) 2.5. Heat Loss in Exhaust An estimate heat loss to the exhaust can be made by measuring the difference between the exhaust and ambient temperatures (25 o C) and assuming a typical value of 1 kj/kg.k for the specific heat of the exhaust gases. 3. THE ENGINE TEST RIG A commercial four cylinders, four stroke, naturally aspirated, water-cooled direct injection compression ignition engine with a 89 kw maximum power at 3200 rpm engine speed and has 295 Nm maximum torque at 1800 rpm engine speed is going to be used to conduct engine performance test. Technical specifications of engine were presented in Table 1. Table 1. Technical specifications of the test engine
Figure 6. The engine test rig 3.1.Measuring Torque The engine torque was measured by the help of a hydraulic dynamometer. S type load cell is used to measure the torque of dynamometer (Stainless steel which has a sensivity of 1/3000) Dynamometer control unit was used for receiving and collecting all the data that was used by the system. Table 2. Technical specifications of the dynamometer 3.2.Measuring Speed The speed sensor used to detect prime mover speed is the magnetic pickup (MPU). When a magnetic material (usually a gear tooth driven by the prime mover) passes through the magnetic field at the end of the magnetic pickup, a voltage is developed. The frequency of
this voltage is translated by the speed into a signal which accurately depicts the speed of the prime mover. 3.3. Measuring Exhaust Gas Temperature Exhaust gas temperature is measured by a thermocouple. The thermocouple is located in the exhaust pipe closed to the cylinder block of the engine. 3.4. Fuel Measurement Fuel quantity is measured in every 5 seconds by a load cell unit. Then, fuel consumption (g/s) is calculated with the aid of a control unit. 4. RESULTS and DISCUSSIONS 1) Plot bmep, torque, brake power, specific fuel consumption and exhaust temperature versus engine speed. 2) Plot ideal and actual power output versus engine speed. Discuss the differences. 3) Discuss the all graphs. 4) Discuss the differences between diesel and gasoline engine. 5) Derive thermal efficiency of diesel engine.
Rpm Torque P B P B,ideal bmep Sfc Thermal Heat (g/s) (g/s) (Nm) (kw) (kw) (kpa) (g/kwh) efficiency loss in exhaust